The present invention relates to an RS-232 serial interface circuit for transmitting and receiving serial data and, more particularly, to an interface circuit including a DC/DC converter circuit which generates positive and negative voltages through a single power supply.
Conventional DC/DC converter circuits, which are constituted as the power supply within an interface circuit of a single power supply system, have consisted mainly of a charge pump system which generates stepped-up positive and negative voltages by switching the charging/discharging timing of a capacitor.
FIG. 1 illustrates a typical arrangement of a conventional DC/DC converter circuit of charge pump system, which comprises: a transmitting circuit OSC for generating a clock signal as a switching signal; an inverter INV for inverting the switching signal generated by the transmitting circuit OSC; an input terminal IN connected to power supply (not shown); a positive and negative output terminals OUT1, OUT2; a charge pumping capacitor C1, a step-up charging capacitor C3 and transistor switches T1, T2, T5 and T6, which constitute a step-up circuit for stepping-up the input voltage; capacitors C2, C4 for stepping-up the positive and negative voltages; and transistor switches T3, T4, T7, T8 constituting a polarity inverting circuit.
In operation of the DC/DC converter circuit illustrated in FIG. 1, when a communication start signal is entered from the outside system, transmitting circuit OSC generates a clock signal which is directly supplied as the switching signal to a first group of the transistor switches T1 to T4, and also supplied, after inverted by the inverter INV, to switch on/off a second group of the transistor switches T5 to T8.
In this ON/OFF timing, the first group of transistor switches T1 to T4 and the second group of the transistor switches T5 to T8 are switched ON and OFF simultaneously but in the opposite timing to each other.
When the second group of transistor switches T5 to T8 are switched ON, the charge pump capacitor C1 of the step-up circuit is turned to the charging mode. When, in the next cycle, the first group of transistor switches T1 to T4 are switched ON, the T5 to T8 are switched OFF, and the charge pump capacitor C1 is turned into the discharging mode, and the charging capacitors C3, C4 are electrically charged at a positive voltage Vdd resulting from stepping-up the input voltage V.sub.in. Subsequently, if, in the next cycle, T1 to T4 are switched OFF and T5 to T8 are switched ON, the charge pump capacitor C1 is turned into the charging mode, and the charging capacitor C2 of the polarity inverting circuit generates the negative voltage Vss in which the charging voltage Vdd of C4 is inverted in polarity. In this manner, the positive and negative output voltages Vdd, Vss resulting from stepping-up the input voltage Vin are continuously obtained from the DC/DC converter circuit.
Under normal conditions, where the single input DC voltage V.sub.in of the DC/DC converter is set at +5 V, a double voltage +10 V is electrically charged into the charging capacitor C3 by the operation of the step-up circuit, and -10 V is electrically charged into the charging capacitor C2.
The .+-.10 V output DC voltages Vdd and Vss resulting from the aforementioned operations are each supplied as the driving voltage to the transmitting circuit (not shown) for transmitting and receiving serial communication data to and from the output terminals OUT1, OUT2.
When such a DC/DC converter circuit is used as a terminal apparatus, in order to lower the power consumption of the set, a clock of the transmitting circuit OSC is stopped during the non-communication time to stop the operation of the DC/DC converter circuit.
In the hand-held data terminal apparatues such as personal computers or word processors, which are based on being driven by battery, in order to achieve a reduction in power consumption, in other words, the miniaturization and lightening of the apparatus through the reduction of the number of loaded batteries, the driving voltage of the signal system attempted have been made to reduce from the conventional 5 V to about 3 V.
However, the reduction of the voltage involves a reduction of the processing speed of the CPU or memory which constitutes the RS-232 interface circuit and, in particular, the reduction in the access time when the data terminal apparatus is driven, thus impeding the realization of the reduction in the voltage to about 3 V.
Therefore, during driving, if the CPU or memory calls for a high speed processing, then the driving voltage is set to 5 V and, if a slow processing speed is sufficient, it is switched to about 3 V to achieve the reduction in the power consumption.
However, in the DC/DC converter circuit used in the serial interface circuits such as a conventional RS-232 interface circuit or the like, in order to achieve the driving system switching of the above-described driving voltage, a circuit system in which the step-up ratio is fixed is adopted to step-up the driving voltage to twice or three times. As a result, a problem can arise which causes the output DC voltage obtained for the output of the DC/DC converter circuit to differ with the driving voltage (input DC voltage).
In general, the output voltage at the transmitting side, which is defined in the RS-232 interface communication standard, is defined to .+-.5 V at most. When a DC/DC converter circuit having the double step-up ratio is used, if the driving voltage (input DC voltage) is 3 V, then the aforementioned .+-.5 V output voltage cannot be assured due to the effect caused by the temperature characteristic or the load characteristic considering the load of the serial interface circuit.
Therefore, during driving at 3 V, a DC/DC converter having the three times step-up ratio becomes necessary. On the other hand, during driving at 5 V, the DC/DC converter circuit having the double step-up ratio is sufficient to assure the foregoing .+-.5 V output voltage. Further, since the step-up ratio is approximately proportional to the power consumption of the DC/DC converter circuit, in order to achieve the intended reduction in the power consumption, the double step-up ratio DC/DC converter circuit becomes necessary.
However, the step-up ratio of the DC/DC converter circuit of the conventional RS-232 interface circuit is constant, which makes it impossible to solve the trade-off problem of both assuring the .+-.5 V output voltage or decreasing the power consumption.